Needled textile fabrics are normally composed of synthetic organic textile fibers, e.g. polyester, nylon, acrylic, etc., or other synthetic organic fibers, needled together into a consolidated mat. While such fabrics may also be made of natural organic fibers, e.g. cotton, hemp, wool, etc., these natural fibers are capable of being formed into a non-woven fabric of substantial properties by the more traditional process, e.g. felting, and hence, are not usually needled to form a non-woven fabric. Inorganic fibers, on the other hand, and especially glass fibers, are not normally either felted or needled, but are consolidated into a fabric of, generally, low physical properties by an air lay or wet lay process. This is because inorganic fibers, by virtue of the materials and process of producing, have very little crimp therein, are of high modulus and substantially brittle, all of which do not lend the inorganic fibers to being either carded, needled or felted. U.S. Pat. No. 3,608,166, for example, details the difficulties in needling glass fibers. Limited needling has been done, however, for the purpose of lightly tacking a glass fiber batt.
U.S. Pat. No. 3,338,777 teaches that the ability to needle glass fibers can be improved by crimping those fibers, but this is quite unacceptable from a commercial point of view because of the cost thereof.
Thus, most needled fabrics, being composed generally of synthetic organic fibers, find a variety of applications where relatively high physical properties are required, e.g. high strengths, with substantially uniform physical properties in both the longitudinal and widthwise direction, and particularly in those applications where economics dictate the use of materials less expensive than woven fabrics or where the applications require more uniform thickness direction properties than woven fabrics, e.g. as a filter media or as a heat insulator. However, since such needled fabrics are generally restricted to synthetic organic fibers, the application of these needled fabrics has been substantially limited when higher temperatures are involved. In addition these needled fabrics have also been limited in applications where filtration requirements are such that the synthetic organic fibers are not of sufficiently small denier to achieve high particulate filtration. Thus, the normal needled fabrics suffer from considerable disadvantages in these regards.
The art has attempted to overcome these disadvantages by use of a number of different approaches. In one approach, high temperature and finer denier synthetic organic fibers have been used to produce the needled fabric, but this solution results in only improved properties, based on the properties of the fibers used, and does not avoid the difficulties associated with the use of synthetic organic fibers, as explained above. For example, the finest denier synthetic organic fiber commercially available in the United States is 1.5 denier. In addition these high temperature and finer denier organic fibers are expensive and their use cannot be accepted in many commercial applications.
Another approach in the art, as exemplified by U.S. Pat. No. 3,338,777, has been that of mixing together organic fibers and glass fibers. Such a mixture of fibers significantly improves the ability to needle the glass fibers into a consolidated fabric of reasonable physical properties. However, this approach suffers from the disadvantage that the mixture of organic fibers and glass fibers decreases the resulting physical properties of the resulting fabric, as opposed to an all organic fiber fabric, due to the increased difficulty of needling the glass fibers into a fabric of high strengths. In addition this approach does not solve the filtration problem. For example, finer particulate filtration is achieved when there is provided a relatively homogeneous layer of finer diameter fibers, e.g. glass fibers, than when such a layer is a mixture of such finer diameter fibers and large diameter fibers, e.g. conventional synthetic organic fibers.
Aside from the foregoing difficulties in resulting properties, a needled mixture of organic fibers and glass fibers has recently been determined to have a most undesired health problem. In needling the mixture, glass fibers may be disposed throughout the thickness of the needled fabric, and including the surfaces thereof. The co-needling of the organic fibers and glass fibers, nevertheless, breaks many of the low stretchable and brittle glass fibers. These very small broken glass fibers are easily displaceable from the surface of the fabric when the fabric is in use and the displaced broken (as well as unbroken) glass fibers will freely float in the air. If workers inhale these broken glass fibers, serious lung damage can result. Accordingly, for safety sake, use of such fabrics is considerably discouraged, or even prohibited, in many industries.
None of the above approaches in the art have produced satisfactory results, and, generally speaking, needled fabrics are only normally composed of organic synthetic fibers, and these fabrics have limitations on their application, as explained above.
An attempt at a different approach is disclosed in U.S. Pat. No. 3,608,166, where organic fibers are used to needle "connecting" fibers through a glass fiber mat, preferably reinforced with a woven fabric, but that patent recommends oiling the glass fibers to avoid the problem of breakage and teaches using only about 12 to 15 needle punches per square centimeter. This is an unacceptably low number of needle punches and the resulting needled mat has low strengths.
A somewhat related but different approach to that of U.S. Pat. No. 3,608,166 is disclosed in U.S. Pat. No. 3,975,565. That patent acknowledges that inorganic fibers are difficult to needle due to the relatively low stretch properties and brittleness of those fibers but goes on to disclose that it was found that inorganic fibers could be satisfactorally needled into a fabric by needle punching a combination of a mat of inorganic fibers and a web of organic fibers when the web of organic fibers is of a small thickness compared to the thickness of the mat of the inorganic fibers. During the needling process, the organic fibers are needled into the body of the mat of inorganic fibers and therefore hold the mat of inorganic fibers to the thin web of organic fibers. This provides a layered product with one layer being that of the inorganic fibers and one thin layer being that of the organic fibers. Thus, the properties of the layer of the inorganic fibers are substantially retained, e.g. the fine particulate filtration properties, and the layer of the organic fibers serves to hold the layer of inorganic fibers in place. This approach, therefore, offered a promise of a solution of one of the problems in the art, as identified above.
However, this approach suffers from a decided disadvantage. In practice, it was found that in needling the organic fiber layer to the inorganic fiber layer, needle punches of more than 260 punches per square inch could not be tolerated, since otherwise damage to the inorganic fibers occurred and that with needle punches of more than 260 per square inch, inorganic fiber damage resulted in more than a 25% loss of the inorganic fiber layer strength. Such a low number of needle punches per square inch is not capable of producing high overall physical properties of the composite. Further, such a low number of needle punches is not capable of so needling the fibers as to produce uniform properties in the needled layers, since most of the fibers in the layers will retain the original laid orientation, as opposed to the highly entangled orientation, and hence high strength and uniform properties, achieved by high numbers of needle punches.
In addition, the considerable breakage, during needling, of the inorganic fibers, constitutes a very definite health hazard, as explained above in connection with the needled combination of organic fibers and glass fibers.
Thus, while in principle the process and product described in that patent offered promise, in practice, the low number of needle punches acceptable and the breakage of the inorganic fibers resulted in much less than a desired solution to the problem in the art.
Accordingly, it would be of substantial advantage to the art to provide a needled fabric made of a combination of a glass fiber layer and an organic fiber layer which can nonetheless be needled to high numbers of needle punches per square inch, to achieve the physical properties discussed above, while at the same time not presenting a health hazard in use of such product.